This invention relates to a process for producing a toner for electrophotography utilizing a polymerization process.
In electrophotography, a photosensitive material is charged uniformly with electricity, and the charged material is then exposed to an optical image formed to make extinct or decrease the charge on the part of the material irradiated by light and thereby to form an electrostatic latent image on the photosensitive material, and thereafter the latent image is developed with a developer containing a toner. The toner image thus developed is generally transferred to an appropriate transferring material and then fixed to form a so-called copy.
The developer used in the above-mentioned process basically comprises, as the principal components, a colorant for developing the electrostatic latent image and a binder for adhering the developed image to the transferring material. These developers are divided broadly into so-called wet (liquid) developers and dry developers.
The dry developers can be further divided into two-component developers and one-component developers. The former comprises a carrier and a toner, and the latter comprises a toner alone. In other words, two-component developers are those wherein toners having a polarity reverse to the electrostatic image required for developing the electrostatic image on the photosensitive material are obtained by triboelectric charging between the carrier and the toner, whereas one-component developers are those wherein the necessary charge is obtained by mutual friction of toners or friction between the toner and other parts of the developing machine.
Up to now, such toners for dry developers have generally been produced by a process which comprises melt-kneading a colorant such as carbon black and/or a magnetic powder such as magnetite powder into a thermoplastic resin to form a disperse material, then grinding said disperse material into particles of desired diameters by applying mechanically an impact force to the material by means of a suitable grinding apparatus and further, if necessary, subjecting the ground material to classification to obtain toners (this process is hereinafter referred to as "grinding process").
Further, in Japanese Patent Appln Kokoku (Post-Exam Publn) No. 10799/68, there has been proposed a process for producing perfectly spherical toner particles by spray-drying an emulsion obtained by emulsion polymerization.
Further, as to processes for producing toners utilizing a polymerization process for overcoming the difficulties of the grinding process, there have been proposed in Japanese Patent Appln Kokoku (Post-Exam Publn) No. 14895/76 and Japanese Patent Appln Kokai (Laid-Open) No. 53756/82 process for producing toners by suspension polymerization. Perfectly spherical toners can be obtained in processes utilizing suspension polymerization.
However, the grinding process requires a great amount of energy in melt-kneading and grinding. Moreover, the toner produced by the process has inevitably many defects.
Particularly, when a resin favorable for the melt-kneading step and the grinding step, for example an easily meltable resin, is employed, it causes cohesion (caking) of the toner during storage or fogging due to toner filming on the photosensitive material. Further, when an easily pulverizable resin is used, the toners are pulverized in the developing machine into fine toners, causing fogging of images and stains of the inside of the developing machine.
Moreover, the colorant dispersed in the resin tends to emerge to the surface of the pulverized toner. This gives rise to decrease in the quantity of tribo-electric charge under high humidity conditions and falling off of the colorants in the developing machine. These in turn cause such unfavorable phenomena as stains of the carrier surface and stains of the surface of the photosensitive material.
On the other hand, it has been revealed that although toners obtained by utilizing emulsion polymerization-spray drying or suspension polymerization can overcome several of the difficulties of toners obtained by the grinding process, they bring about new difficulties. Namely, since the particles of toners thus obtained are perfectly spherical, the toners have a poor cleaning property. Further, since emulsifiers or suspending agents remain in the toner particles, the toners have decreased charge stability and decreased caking resistance.
In the meantime, as to the methods for fixing the electrostatic image in electrophotography, there have been known various methods including heated roll methods, pressure fixing methods, high-frequency heating methods and flash methods. The heated roll methods are most commonly used at present.
The heated roll methods include an oil coating method wherein a release agent such as silicone oil is coated on the roll surface, and an oilless method wherein a release agent such as silicone oil is not used and the fixing is effected by means of a roll using a material excellent in release property such as Teflon and silicone rubber. In both cases, there occurs a problem of offset phenomenon wherein the toner molten by heat transfers to the heated roll and stains the image-holding material such as paper.
Various methods have been proposed to prevent this offset phenomenon. One of the proposed methods comprises adding to the toner a resin comprising polyolefin as the principal component in order to improve the release property of the toner.
Also in the case of producing toners by suspension polymerization, there is known a process wherein an offset prevention agent is added in polymerization, as described in Japanese Patent Appln Kokoku (Post-Exam Appln) No. 13731/84 that ". . . comprising a step of polymerizing a monomer, which gives after polymerization a polymer which is a constituent of a dry type toner for heated-roll fixing type electrostatic image, in the presence of an offset prevention agent".
However, this process gives a polymerization liquid containing a number of agglomerates since the hydrophobic/hydrophilic balance of the polymerization system is lost owing to polyolefins of the offset prevention agent. Further, since polyolefin is more hydrophobic than styrene-acrylic resin which is a constituent of the toner, the former is localized in the core part of toner particles and scarcely present on the particle surface or in the vicinity thereof, so that offset prevention effect is not satisfactorily exhibited.